Protective apparatus and system



Dec. 25, 192s. 1,696,289

J. D. WOOD PROTECTIVE APPARATUS AND SYSTEM Filed July 24, 1925 l I Zine curfew? l avefzoaa m7, e Jveo', /VarmaIwor-A''n run e W O* l 0 /TZPWHQ@ g Patented Dec. 2'5, 1928.

UNITED STATES JOSEPH D. WOOD, F WOLLASTON, MASSACHUSETTS, ASSIGNOR, BY MESNE ASSIGN- i MENTS, ,T0 CONDIT ELECTRICAL MANUFACTURING CORPORATION, OF

0F MASSACHUSETTS. v

MASSACHUSETTS, A CORPORATION 1,696,289 PATENT OFFICE.

BOSTON,

PROTECTIVE APPARATUS AND SYSTEM.

Application med July 24, 1925. Serial No. 45,936. l

This invention relates to protective apparatus and systems which include a thermal relay having an expansible element which is adapted to be heated upon a predetermined rise of current in an electric circuit to effect the interruption o the circuit.

The thermal relay is usually, although not necesarily, of the type having an' expansible circuit-controlling element composed of two metals having dissimilar expansion coeiicieiits, and an electric heating element for the expansible element.

The heating element is ordinarily arranged to be influenced by the current in the circuit with which the relay is associated and an increase of current in the circuit results in a proportionate increase of current in the heating element of the relay. There is some time lag between the hea-ting of the heating element and the expansion of the expansible element and the consequent interruption of the circuit. Under certain conditions of operation, when a heavy overload occurs on the circuit with which the relay is associated, the increase of current in the heating element is so great as to burn out or damage the heating element before sufficient time has lapsed to enable the thermal element to operate an interrupting circuit. As a consequence, it has been the general practice to interpose a fuse in the circuitof the heating device whereby to protect it from damage due to such excessive currents, the fuse being adapted to blow when the current in the heating element reaches a value which becomes dangerous to the heating element, The use of a fuse, however, is not desirable since it increases the cost of the apparatus and occupies space which might otherwise be more advantageously utilized.

An object of this invention is the provision of means to prevent damage to the thermal relay or to the heating element thereof when the circuit with which the relay is associated is traversed by heavy overloads.

A yet further object of the invention is the provision of means to prevent the passage of a damaging amount of current through the heating element of the thermal relay, regardless of the value of the overload in the power circuit and While maintaining the relay effective at all times to interrupt the power circuit when it is excessively loaded.

A further object of the invention is generally to improve the construction and operation of protective apparatus and systems and thermal relays. In carrying out the invention, the heating element of the thermal relay is supplied with current from. the secondary of a current transformer, the primary of which istincluded in the owercircuit to be protected by the relay. e transformer has such characteristics that, or the normal load range in the power circuit, there will be a more or less proportionate range of current in the secondary and the heating element, but when the power circuit becomes seriously overloaded the iron core of the transformer is adapted to become saturated so that, regardless of high current values in thepower circuit, the current in the secondary and in the heating element remain practically constant and at a value which will not cause 'damage to the heating element of the thermal relay within the time required for the relay to operate and interrupt the circuit.

Fig, 1 is a diagram illustrating the application of the invention to the control of an electric power circuit.

Fig. 2 is a longitudinal sectional elevation through a thermal relay well adapted for usein connection with the invention.

Fig. 3 is a section along line 3-3 of Fig. 1 and illustrating more particularly the normal arrangement of the contact members of the thermal relay.

Fig. 4 is a diagram illustrating the ratio of secondary current to rimary current in the relay heating trans ormer which is a feature of this invention.

The invention is here applied to the control o an electric circuit 10 which supplies electric energy to some translating device as an electric motor 12 under the control of an automatically opening switch or circuit breaker 14. The protective apparatus embodying the invention includes a thermal relay which as here shown comprises an insulating tubular enclosing casing 16 having metal ferrules 18 and 20 on the ends thereof. An electrical heating element or grid 22 extends through the interior of the casing and is electrically connected with the end ferrules. Said heating element is here shown as a cylindrical rod of suitable resistance material although the heating element may be of any desired material and may assume any desired configuration, the physical characteristics of the heating element being unimportant to this invention. Ordinarily, however, the heating element is ar `anged to require but a small amount, as one ampere or thereabouts, of current, to effect the operation of the relay.

A thermally expansible vmember 24, comprising a spirally-formed strip of two metals having dissimilar expansion co-efiicients united together, is disposed about said heating element 22 and in heat receiving relation therewith although preferably free from actual contact t-herewith. One end of said spiral eXpansible element is secured to one of the end caps as the end cap 18, and is electrically connected with said end cap. The other end 26 of said expansible. element is free and is adapted to move under the influence of heat. A ixedly-supported, although preferably adjustable, contact member 2S is disposed to be disengaged by the free end 26 of the expansible element whereby to effect the opening of the circuit breaker 14 under predetermined excess current conditions in the power circuit 10.

The heating element 22 of the relay is electrically connected with the secondary winding 30 of a current transformer 32, the primary winding 34 of which current trailsformer is in series with the power circuit 10 and preferably in that portion of the circuit included between the motor 12 and the circuit breaker 14 so that, when said circuit breaker is open, the current transformer is de-energized. Since ordinarily the heating element 22 is adapted to require but a small amount of current to operate the expansible element 24 while the amount of current normally flowing in the circuit 10 may be much greater, the current transformer 32 is arranged'to step down the current between the primary and the secondary thereof.'

The relay here shown is arranged to control a normally closed protective circuit so that under normal load conditions in the circuit 10, the relay contacts 2G and 2S are in contact. The relay is arranged to control the circuit of an electro-magnet 8G which is associated with the circuit breaker' 14 and said coil is connected across the power circuit between the motor 12 and circuit breaker 14 and through the relay as shown in Fig. 1. Under normal current conditions in the power circuit l0, the coil 36 is normally energized by the potential of said circuit and thereby maintains its armature or plunger 38 in elevated position. `"When an abnormal current flows through the power circuit, the transformer 32 serves to heat the heating element 22 and thereby actuate the expansible element 24 of the relay to open the circuit of the coil 36. The plunger 38 thus falls by gravity and engages the latch 40 of the circuit breaker and to trip open the circuit breaker and thereby effect the interruption of the power' circuit.

Due to the method of heating the expansible element 24 of the relay by transferring heat to it from Athe heating element 22, a eertain amount of time lapses between the heating of the heating element and the sufficient expansion of the expansible element to effect the consequent interruption of the power circuit.

As heretofore arranged, a sulliciently Vheavy overload in the power circuit resulted in heating the heating element 22 so highly as to fuse it or otherwise damage it before-sufficient time had lapsed to enable the.expansible element 24 to expand and open the power circuit. In the present invention,the system and the apparatus therein contained is so arranged that excessive heating of the heating element upon an excessive overload in the power circuit can not take place during the time that the expansible element 24 becomes heated and operates to open the power circuit.

In accordance with this invention, this result is attained by so designing the current transformed 32 that the secondary current therein which flows through the heating element will not be enough greater, upon excessive overloads, thant the normal secondary current due to normal overload conditions in the power circuit, as to,exeessively heat and damage the heating element 22 within the time required for the relay to operate to open the circuit breaker 14. To this end, the current transformer 32 is provided with an iron magnetic core of relatively small cross-section so that the core becomes saturated for excessive currents in the power circuit and thereby provides a transformation curve a illustrated in F ig. 4 wherein the abscissa represents the line current or the current in the primary winding 34 of the transformer and the ordinate represents the current in the secondary 30 of the transformer and in the heating element 22 of the relay. For normal current conditions in the power circuit 10, the core of the transformer is designed to be magnetized below its saturation point and so give the transformation characteristic illustrated by the first part, or the part l), of the curve a, wherein the secondary current varies more or less in proportion to the variation ol' the primary current. At some predetermined value of current inthe primary winding 'S-l indicated by the abscissa the core of the transformer is adapted to become saturated so that for primary or line currents in excess of this value as indicated by the part c of the curve a, the current in the secondary coil and the heating element 22 will not be increased to a harmful degree regardless of the increase in value of the line curlll) rent. The heating element 22 is adapted to 1vwithstand this relatively small increase of regardless of the current in the power 'circuit, and while maintaining the relay constantly in operative condition.

I claim:

1. Protective`apparatus for electric circuits including a thermally-actuated, circuit-controlling rela having an electrical heating element, anv means to keep down the temperature of said heating element below a destructive value regardless of the strength of current in the circuit including a transformer to furnish energy for heating. said heating element energized by the current in the electric circuit and characterized by limiting the current in the heating elenient at a predetermined value regardless of the degree of energiz'ation of the transformer.

2. Protective apparatus for electric cir-- cuits including a thermally-actuated circuitthe transformer is excessivel value, and thereafter to controlling relay having an electrical heating element, and means to keep down the temperature of said heating element below a destruct-ive value regardless of the strength of current in the circuit including a transformer to furnish venergy for heating said heating. element energized b the current in the electric circuit and c a-racterized by having a core which becomes saturated when energized;

3. Protective apparatus or an electric circuit includingl avthermally-actuated, circuit-controlling rela having an electrical heating element, an means to keep down the temperature of said heating element below a destructive value regardless of the strength of current in the circuit including a transformer to furnish energy for heating said heating element from the circuit, said means to pass current through said heating element approximately proportional to the current in t-he circuit, up to a predetermined the heating element approximately at that value regardless of further increases in value of the current in the circuit. y

4. Protective apparatus for electric circuits including a thermally-actuated,A circuit-controlling relay having an v electrical heating element, and a current-limitingcur-- rent transformer for heating said heating element electrically. 5. lProtective apparatus for electric circuits including a thermally-actuated, circuit-controlling relay having an electrical hold the current in' heating element and a current-limiting transformer for heating said heating element electrically having a core adapted to become saturated at a current value in said heating element which is below a value destructive to said heating element, whereby to prevent damage to said heating element. "6. Protective apparatus for electric circuits including a protective relay having a thermally-expansible element, an electrically heated heating element for said expansible element, and a current-limiting transformer. to `furnish energy for heating said heating element having a core adapted `to become saturated at a current value in said heating element which is below a value destructive to said heating element, whereby to prevent damage to said heating element.

7. Protective apparatus for electric` circuits including a protective relay having an electric heating element and a thermally-expansible element actuated with a time lag by the heating of said heating element, and means to heat said heating element electrically from a variable current source and to keep down the temperature of said heating elementv including a transformer having a primary windin in the circuit to be protected and a secon ary winding conc tinuously connected with y said heating element and a core adapted to become saturated upon -overloads, whereby to keep down the current in said heating element. 8. Protective apparatus for electric circuits including a protective rela havingan electric heating element and a rmallyexpansibleelement actuated with a'. time lag by the heating of said heating element, a transformer to furnish energy for heating said heating element electrically from the electric circuit and having means to keep down the temperature of said heating element including a magnetic core `adapted to become ysaturated on.A -overloads whereby to keep down the current in said heating element' below a predetermined value.

9. An overload protective device for an 'electric circuit having a current transformer energized by said circuit,'and an overload-protective-tripping relay secondary of the transformer and from the for the circuit energized adapted the combination of to operate whenever a. predetermined valuer t of current exists vin e secondary for a sufficient 'periodici i time,v said transformer having a core adapted to become saturated on 'overloads in the circuit above a certain keep down'the currentin that will not value, whereby to the tripping relay damage it. l' i y In testimony whereof, I have signed my name to this specification.

. JOSEPH D. woop.

to .a value 

